[flang-commits] [flang] [flang][docs] add design doc for inline runtime checks (PR #228502)
Eugene Epshteyn via flang-commits
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Thu Oct 8 15:51:59 PDT 2026
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@@ -0,0 +1,785 @@
+<!--===- docs/InlineRuntimeChecks.md
+
+ Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ See https://llvm.org/LICENSE.txt for license information.
+ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+-->
+
+# Inline runtime checks in Flang with `fir.assert`
+
+Status: draft design, for discussion.
+Context: [llvm/llvm-project#223287](https://github.com/llvm/llvm-project/pull/223287).
+Source references are to `llvm-project` at `fe1de64cb3c2` (2026-10-02). Line
+numbers drift, so each reference also names the function or pattern.
+
+Abbreviations used in the tables:
+
+| Short | Path |
+|---|---|
+| `SHI` | `flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp` |
+| `OB` | `flang/lib/Optimizer/HLFIR/Transforms/OptimizedBufferization.cpp` |
+| `IHA` | `flang/lib/Optimizer/HLFIR/Transforms/InlineHLFIRAssign.cpp` |
+| `SAA` | `flang/lib/Optimizer/HLFIR/Transforms/SeparateAllocatableAssign.cpp` |
+| `C2F` | `flang/lib/Optimizer/HLFIR/Transforms/ConvertToFIR.cpp` |
+| `SI` | `flang/lib/Optimizer/Transforms/SimplifyIntrinsics.cpp` (FIR level) |
+| `IC` | `flang/lib/Optimizer/Builder/IntrinsicCall.cpp` |
+| `X2H` | `flang/lib/Lower/ConvertExprToHLFIR.cpp` |
+| `RT/` | `flang-rt/lib/runtime/` |
+
+Terminology:
+
+- **Lowering** is the translation of the Fortran parse tree to HLFIR, in
+ `flang/lib/Lower`.
+- **Emitting** an assert means creating a `fir.assert` operation, either in
+ lowering or in an HLFIR or FIR pass.
+- **Code generation** is the conversion of FIR to the LLVM dialect
+ (`FIRToLLVMLowering` in `flang/lib/Optimizer/CodeGen/CodeGen.cpp`), where
+ `fir.assert` becomes a branch and a runtime call.
+
+## 1. Problem
+
+Flang relies on the Fortran runtime for almost all dynamic checks: shape
+conformance, allocation status, argument consistency of intrinsics. Every
+time an HLFIR or FIR pass replaces a runtime call with inline code, the check
+silently disappears. Three consequences:
+
+1. **Behavior depends on the optimization level.** The inlining passes mostly
+ run at `-O1` and above, so the same non-conforming program stops with a
+ clear message at `-O0` and does something else at `-O2`.
+2. **The inline code is often memory-unsafe on non-conforming input, not just
+ wrong.** Loop bounds are taken from one operand and used to index another
+ (`fir::factory::deduceOptimalExtents`). A size mismatch becomes an
+ out-of-bounds read or write instead of an error.
+3. **Some checks the standard requires are missing on every path.** For
+ example, the inline ALLOCATE/DEALLOCATE of intrinsic-type allocatables
+ does not check the allocation status (F2018 9.7.1.3, 9.7.3.2, 9.7.4).
+
+The few inline checks that exist today (MOD/MODULO `P==0`, NEAREST `S==0`,
+STORAGE_SIZE, the realloc-with-scalar-RHS case) use `fir.if` plus
+`fir.call @_FortranAReportFatalUserError`. That pattern:
+
+- has no data dependence on the operation it protects, so passes may move the
+ protected operation above the check (MLIR models neither "may not return"
+ nor non-local control flow: `mlir/docs/Rationale/SideEffectsAndSpeculation.md`);
+- introduces a call with unknown memory effects, which blocks alias-based
+ optimizations;
+- does not tell LLVM the call never returns (the FIR declaration has no
+ `noreturn`), so LLVM cannot use the checked condition afterwards.
+
+## 2. Goals and non-goals
+
+Goals:
+
+- Keep every user-facing check the runtime performs when the call is inlined,
+ and report it with the runtime's message format,
+ `fatal Fortran runtime error(<file>:<line>): <message>`, and with message
+ text as close as possible to the runtime's.
+- Make non-conforming programs behave the same at `-O0` and `-O2`, so that
+ tests disabled because of that difference (for example in the
+ llvm-test-suite gfortran tests) can be re-enabled.
+- Keep the checks cheap and transparent to optimization. They must not block
+ store-to-load forwarding, CSE of loads, LICM of loads, or alias analysis.
+- Let checks be hoisted, merged, folded, and dropped under an option.
+- Provide one mechanism for future opt-in checks (`-fcheck=bounds`,
+ `pointer`, `do`, `bits`).
+
+Non-goals:
+
+- Detecting undefined POINTER association status (only disassociated, that
+ is null, is detectable).
+- Checking explicit-shape dummy size against the actual argument in general
+ (the callee only receives an address).
+- Changing the runtime's own checks, apart from the bugs listed in the
+ appendix.
+
+## 3. The `fir.assert` operation
+
+### 3.1 Semantics and syntax
+
+```
+%g:N = fir.assert %ok, <kind>, "<message>" [values(%v0, ... : T0, ...)] [guard(%x0, ... : U0, ...)]
+```
+
+- `%ok` (`i1`) is the condition that must hold.
+- `<kind>` is a check category (section 4). Options act on categories.
+- `<message>` is the error text, a `printf` format.
+- `values(%v0, ... : T0, ...)` are optional integer or `index` SSA values
+ (`T0`, ... are their types) printed into the message, one per `%jd`
+ directive. They are only read on the failure path. For example, the two
+ extents in `"DOT_PRODUCT: SIZE(VECTOR_A) is %jd but SIZE(VECTOR_B) is %jd"`.
+- `guard(%x0, ... : U0, ...)` are the optional guarded values, of any type.
+ The op has one result per guarded value, with the same type (`%g#i` has
+ type `Ui`).
+- If `%ok` is true, each result equals the corresponding guarded value and
+ nothing else happens. Otherwise the program terminates through the Fortran
+ runtime with the formatted message.
+- Operations that must not run before the check consume the guarded results.
+ The ordering is then enforced by SSA dominance, for every pass, without any
+ pass having to know about `fir.assert`. Asserts without a guard only order
+ themselves with respect to I/O and calls (section 3.2).
+
+What to guard: the operand of the protected operation, such as a divisor,
+a DO step, a subscript, or a box that is dereferenced. For conformance
+checks, it is the **array being accessed** (a variable, or an
+`!hlfir.expr`), at the point where it is combined with another array:
+
+- In an elemental operation or an elemental procedure reference, guard the
+ array operands that do not provide the iteration space.
+- In an array assignment, guard both sides. The pass that expands the
+ assignment may iterate over either one.
+
+Conformance checks do not guard shapes or extents. HLFIR can re-derive them
+without going
+through a guarded value: from static types (`hlfir.shape_of` folds to a
+constant shape), from the preference for constant extents
+(`fir::factory::deduceOptimalExtents`), or from the other side of an
+assignment. Leaving them unguarded also keeps constant loop bounds visible
+to MLIR loop optimizations. Array bases are never compile-time constants, so
+guarding them loses nothing. Section 7 explains how the guards follow an
+expression when it is inlined.
+
+### 3.2 Traits
+
+- `MemoryEffects<[MemWrite<FortranRuntimeResource>]>`, where
+ `FortranRuntimeResource` is a non-addressable resource (like the existing
+ `fir::DebuggingResource`).
+ - The op is never trivially dead, so DCE keeps asserts without uses.
+ - It stays ordered with calls and I/O, whose unknown effects cover every
+ resource.
+ - It does not touch user memory. FIR alias analysis skips non-addressable
+ resources (`flang/lib/Optimizer/Analysis/AliasAnalysis.cpp`, `getModRef`),
+ and MLIR CSE ignores writes to resources disjoint from a load's resource.
+ So asserts do not block load CSE, store-to-load forwarding, or FIR LICM of
+ loads. This also means `SimplifyHLFIRIntrinsics` can emit asserts in its
+ first run (`allowNewSideEffects=false`, `SHI:3321-3333`): the concern
+ there is new effects that stop CSE from merging loads, which this op does
+ not have.
+- Not speculatable.
+- **Not transparent to other operations' folds and patterns.** The assert's
+ own canonicalization removes it when `%ok` is a constant true (section 3.4).
+ But no other rewrite may look through an assert to its guarded operand,
+ because that drops the dependence. For example, in
+ `%a = fir.box_addr %b; %a_ok = fir.assert ... guard(%a); %c = fir.embox %a_ok`,
+ a fold that rewrites an `fir.embox` of a `fir.box_addr` into the original
+ box must not replace `%c` with `%b`. In practice:
+ - the op does not implement `ViewLikeOpInterface`;
+ - it is not added to the helpers that skip `fir.declare` or `fir.convert`
+ to find a defining op;
+ - it is not given a folder that returns a guarded operand when `%ok` is
+ unknown.
+
+ Alias analysis may look through the assert, since it only answers
+ aliasing queries and does not rewrite uses. So may the analyses that read
+ Fortran attributes or contiguity from a guarded variable's
+ `hlfir.declare`.
+
+### 3.3 TableGen sketch
+
+```
+def fir_AssertOp : fir_Op<"assert", [AttrSizedOperandSegments,
+ MemoryEffects<[MemWrite<FortranRuntimeResource>]>]> {
+ let arguments = (ins I1:$condition,
+ fir_RuntimeCheckKindAttr:$kind,
+ StrAttr:$message,
+ Variadic<AnySignlessIntegerOrIndex>:$values,
+ Variadic<AnyType>:$guarded);
+ let results = (outs Variadic<AnyType>:$results); // types == guarded types
+ let hasVerifier = 1;
+ let hasCanonicalizer = 1;
+}
+```
+
+### 3.4 Canonicalization and redundancy elimination
+
+- **Constant true:** erase the assert and replace its results with the guarded
+ operands. This replaces the hand-written `fir::getIntIfConstant` early-outs
+ of today's checks (`IC:6929-6931`). It must be a canonicalization pattern
+ rather than only a `fold`: for an assert with no guard, an empty fold result
+ means an in-place update, not an erasure.
+- **Constant false:** keep it, with no diagnostic. A false condition is common
+ in code that is unreachable but hard to prove unreachable at compile time
+ (for example, after inlining or specialization on constant arguments), so a
+ warning would be noisy.
+
+ A later pass could erase the code strictly dominated by a constant-false
+ assert. This is not proposed for the first version, and it must not run in
+ any pipeline that may drop asserts (for example, device code with checks
+ disabled). There, the assert would disappear together with the pruned
+ code, and a program that should have stopped would silently produce
+ wrong results.
+- **Dominating assert with the same condition and kind:** remove the later
+ one, mapping its results to the earlier one's results. This is valid
+ regardless of what lies between them and regardless of the message, because
+ the earlier check fails first. Two restrictions:
+ - The kinds must be the same. Some pipelines drop asserts by kind after
+ this point (section 4), and removing a check because of an earlier one
+ that is dropped later would lose it.
+ - The earlier assert must guard everything the later one guards; otherwise
+ the consumers would lose their dependence.
+
+ Upstream CSE cannot do this: it skips ops with write effects
+ (`mlir/lib/Transforms/Utils/CSE.cpp`, `simplifyOperation`). So this needs a
+ small dominance-based pass (section 6). It is not required to start with:
+ once the runtime call is declared `noreturn`, LLVM already removes many
+ redundant checks (GVN, jump threading). The MLIR pass is for cleaning up
+ checks early, before the MLIR loop analyses and transformations that would
+ otherwise have to step around them.
+
+### 3.5 Code generation
+
+`fir.assert` is converted in `FIRToLLVMLowering`, where the IR is already a
+control-flow graph:
+
+```
+// fir.assert %ok, conformance, "..." values(%na, %nb : index, index) guard(%b : !fir.box<...>)
+llvm.cond_br %ok weights([2000, 1]), ^cont, ^fail
+^fail:
+ llvm.call @_FortranAReportFatalUserErrorValues(%msg, %file, %line, %na64, %nb64, ...)
+ llvm.unreachable
+^cont: // uses of the result are replaced with %b
+```
+
+- **Runtime entry.** `_FortranAReportFatalUserError(msg, source, line)`
+ (`flang/include/flang/Runtime/stop.h`) is `[[noreturn]]` and device-callable
+ (`RT_API_ATTRS`). It takes no values. Proposed addition:
+ `ReportFatalUserErrorValues(msg, source, line, int64 v0..v3)` with a fixed
+ number of `int64_t` arguments. That keeps the ABI simple on devices, and
+ messages use `%jd` only. The message is passed to `printf`, so a literal
+ `%` in a message must be escaped.
+- **Declaration.** The runtime function declaration must carry `noreturn`,
+ so LLVM can use the checked condition on the continuation path.
+- **Source location.** Take it from the assert's own location. Resolve
+ `FusedLoc` and `CallSiteLoc` to the innermost `FileLineColLoc`: today
+ `fir::factory::locationToFilename` returns null for them, so inlined checks
+ lose file and line.
+- **Device code.** String globals must be created in the nearest symbol table
+ (a `gpu.module` for device code), and the conversion must also exist in
+ any device code generation pipeline. If a device runtime lacks the entry point,
+ fall back to `printf` plus `llvm.trap`. The function is not in the OpenMP
+ offload API group today, unlike `Abort`.
+
+Converting asserts earlier, before `CFGConversion` (to `fir.if` plus call), is
+also possible, but `fir.if` regions cannot end with `fir.unreachable`, and
+asserts would stop being visible to the passes after that point.
+
+### 3.6 Alternatives considered
+
+**`fir.if` plus a runtime call (today's pattern).** Not retained, for the
+reasons in section 1: there is no data dependence on the protected operation,
+the call's unknown memory effects block alias-based optimizations, and LLVM
+is not told that the call never returns.
+
+**`cf.assert`.** Not retained. It writes the default resource, which is how
+it models program termination, so it blocks store-to-load forwarding and
+load CSE across it. It still does not order non-speculatable operations
+without memory effects, such as `arith.remsi`, after it. Its conversion to
+LLVM prints with `puts` and calls `abort`, rather than reporting through the
+Fortran runtime. It also has no guarded results.
+
+**A result-less `fir.assert` (the current revision of
+[#223287](https://github.com/llvm/llvm-project/pull/223287)).** The PR
+defines `fir.assert` as a non-speculatable op with a
+`MemWrite<FortranRuntimeResource>` effect, converts it to `cf.assert`, and
+teaches Flang LICM not to hoist non-speculatable operations across a
+preceding assert in the same block. This design keeps that effect model
+(section 3.2), which gives the right DCE, alias-analysis, and forwarding
+behavior. It is not retained as is, because the ordering of the protected
+operations is not expressed in the IR:
+
+- MLIR has no notion of an operation that may not return. Every pass may
+ assume that all operations in a block execute together, and may move a
+ `fir.load` or an `arith.remsi` above an assert whose effects do not
+ conflict with it. The LICM change protects one pass. Every other pass that
+ moves or recreates operations would need the same fix, and future passes
+ would have to remember it. Examples are sinking, scheduling, and rewrite
+ patterns that build new operations at a different insertion point.
+- Once a protected load or remainder is above the check, LLVM may infer from
+ its undefined behavior that the check cannot fail, and delete it.
+- The guarded results make the ordering a matter of SSA dominance, so no pass
+ has to know about `fir.assert`. Code generation then calls the Fortran
+ runtime entry directly, so messages match the runtime's format (section
+ 3.5).
+
+The design in this document is therefore the PR's op, with guarded results
+added and code generation calling the runtime directly.
+
+**A region-based assert, like `shape.assuming`.** The protected operations go
+inside a region attached to the assert, and the values used afterwards are
+yielded out:
+
+```
+%r = fir.assert %ok, conformance, "..." -> (f32) {
+ // inlined DOT_PRODUCT loop nest
+ fir.result %sum : f32
+}
+```
+
+Ordering is structural, so it is as safe as the guarded-result form. It is
+not retained because the region gets in the way of the optimizations the
+checks must not block:
+
+- The protected code is usually a whole loop nest or the rest of a
+ statement, so everything it defines has to be yielded through the region.
+ An allocation-status check would wrap the rest of the block.
+- Passes that work on a single block or match operation sequences see a
+ region boundary instead. Examples are the `OptimizedBufferization` pattern
+ that matches an `hlfir.elemental` and its `hlfir.assign`, store-to-load
+ forwarding, and LICM of loads out of the region.
+- Merging two redundant asserts means merging or inlining regions. Hoisting
+ one means moving its whole region, or splitting it.
+- Several checks on the same statement nest regions inside each other.
+
+The guarded-result form gives the same guarantee at the granularity of a
+value: guarding the arrays a loop accesses protects every access in the
+loop, without moving the loop into a region.
+
+## 4. Check categories and default policy
+
+| Kind | Meaning | Default | Option |
+|---|---|---|---|
+| `status` | Allocation status required by the standard (ALLOCATE/DEALLOCATE, assignment to an unallocated or disassociated left-hand side) and allocation failure | On, all optimization levels | Option to disable, for diagnosing regressions |
+| `conformance` | Shape or size agreement that the runtime checks when not inlined | On, all optimization levels (same behavior as `-O0`) | `-fno-check=conformance` (name to be decided) |
+| `argument` | Intrinsic argument values the runtime checks (DIM range, RESHAPE SHAPE values) | On | same mechanism |
+| `bounds` | Subscript, section, substring, and character-dummy-length checks | Off | `-fcheck=bounds` |
+| `pointer` | Dereference of a disassociated POINTER, an unallocated ALLOCATABLE, or an absent OPTIONAL | Off | `-fcheck=pointer` |
+| `do` | DO loop with zero step | Off | `-fcheck=do` |
+| `bits` | Bit-intrinsic position and shift ranges | Off | `-fcheck=bits` |
+| `divide` | Integer MOD/MODULO with `P==0` | Off | existing `-fcheck-integer-mod-zero-divisor` |
+
+Gating:
+
+- Asserts are gated where they are emitted. Lowering and the passes that
+ emit asserts do not emit the ones whose kind is disabled, whether it is a
+ default-on kind disabled by an option or an opt-in kind that is not
+ enabled. Disabled checks therefore never affect the MLIR passes, and cost
+ nothing.
+- The enabled kinds reach lowering and the passes through a module
+ attribute, as `-fcheck-integer-mod-zero-divisor` does today
+ (`fir.check_integer_mod_zero_divisor`). A single `fir.runtime_checks`
+ attribute listing the enabled kinds would scale better.
+- Dropping asserts after they are emitted is only needed when one function
+ is compiled for several targets with different policies. For example, for
+ an OpenACC `acc routine`, lowering emits a single `func.func` that later
+ becomes a host and a device variant, and some asserts may have to be
+ dropped or simplified in the device variant. A small pass drops asserts by
+ kind, replacing their results with the guarded operands. It runs only in
+ such pipelines, after the variants are split, not in the normal pipeline.
+- `-fcheck=` currently exists only as a gfortran pass-through in
+ `clang/include/clang/Options/FlangOptions.td`. It would become a Flang
+ option with gfortran-compatible sub-options.
+
+## 5. Checks to emit
+
+Each table gives the condition in IR terms, the runtime message to reuse, the
+insertion point, and what to guard. `ext(x,d)` is the extent of dimension `d`
+(0-based) from `hlfir::genExtentsVector` or `fir.box_dims`. `null(p)` is a null
+test of a `fir.box_addr` result.
+
+### 5.1 Checks lost by HLFIR and FIR inlining (`status`, `conformance`, `argument`)
+
+These checks exist in the runtime today and disappear at `-O1` and above.
+
+#### Assignment and copy
+
+| ID | Operation | Condition (`%ok`) | Runtime message | Inserted in | Guard |
+|---|---|---|---|---|---|
+| A1 | Array assignment, no realloc, whatever the right-hand side (variable, `hlfir.elemental`, or `hlfir.eval_in_mem` such as an inlined MATMUL) | for each `d`: `ext(lhs,d) == ext(rhs,d)` | `Assign: mismatching element counts in array assignment (to %jd, from %jd)` (`RT/assign.cpp:453-461`; `AssignSimple` at `:921-928`) | Lowering, before `hlfir.assign` (`flang/lib/Lower/Bridge.cpp`, `genDataAssignment`, `:5884`) | Both sides |
+| A2 | Assignment to a POINTER or non-realloc ALLOCATABLE left-hand side | `!null(lhs) \|\| size(lhs) == 0` | `Assign: left-hand side variable is neither allocated nor allocatable` (`RT/assign.cpp:371-374`) | `IHA` `InlineHLFIRAssignConversion`; `OB` `ElementalAssignBufferization`, `EvaluateIntoMemoryAssignBufferization` | Left-hand-side box |
+| A5 | Scalar broadcast to an unallocated allocatable (realloc) or disassociated pointer | `!null(lhs)` | `Assign: mismatched ranks (%jd != %jd) in assignment to unallocated allocatable` (`RT/assign.cpp:361-370`), or the existing inline text `array left hand side must be allocated when the right hand side is a scalar` (`MutableBox.cpp:947-954`) | `OB` `BroadcastAssignBufferization` (`:514-612`); it fires even with `realloc` | Left-hand-side box |
+| A6 | Inline (re)allocation for assignment | `!null(newStorage)` after `fir.allocmem` | `Memory allocation failed` / `AssignSimple: allocation failed (stat=%jd)` (`RT/assign.cpp:130`, `:1050`) | `SAA` (runs at every optimization level), `IHA` `InlineAllocatableExprAssignConversion`, `C2F` scalar allocatable path (`:123-150`), all through `fir::factory::genReallocIfNeeded` / `allocateAndInitNewStorage` | New storage address |
+
+Not lost: `InlineCopyInConversion` replaces `ShallowCopyDirect`, whose checks
+are internal, because the temporary is built from the variable's own shape.
+
+Notes on A1:
+
+- It is emitted once, in lowering, rather than in each pass that expands
+ `hlfir.assign` (`InlineHLFIRAssign`, the `OptimizedBufferization` patterns,
+ `BufferizeHLFIR`, the runtime-call path). Those passes only see the guarded
+ sides. `OptimizedBufferization` inlines the right-hand side's elemental
+ body through the guard (section 7), and `tryUsingAssignLhsDirectly` writes
+ through the guarded left-hand side.
+- At `-O0`, the runtime then checks the assignment again, which is harmless.
+- Assignments created by passes (`temporary_lhs`, the ones produced by
+ `SeparateAllocatableAssign`) conform by construction and are not checked.
+- The runtime only compares **element counts**. A `2x3 = 3x2` assignment
+ passes the runtime and gives a wrong result in element order. The inline
+ loop nest would access out of bounds, so the assert must compare
+ **per-dimension** extents. That is stricter than the runtime, and it is
+ exactly F2018 10.2.1.2 conformance. The message then also names the
+ dimension (see the A1 example in section 5.4).
+
+#### Reductions, MATMUL, DOT_PRODUCT
+
+| ID | Operation | Condition | Runtime message | Inserted in | Guard |
+|---|---|---|---|---|---|
+| A7 | DOT_PRODUCT | `ext(a,0) == ext(b,0)` | `DOT_PRODUCT: SIZE(VECTOR_A) is %jd but SIZE(VECTOR_B) is %jd` (`RT/dot-product.h:63-67`) | `SHI` `DotProductConversion::genProductExtent`, before `deduceOptimalExtents`; `SI` DOT_PRODUCT at the call site (the generated body always loops over the first vector) | Both vectors (VECTOR_B is enough for `SI`) |
+| A8 | MATMUL | `ext(a, rank(a)-1) == ext(b,0)` | `MATMUL: unacceptable operand shapes (%jdx%jd, %jdx%jd)` and the vector forms (`RT/matmul.h:272-293`) | `SHI` `MatmulConversion::genResultShape`, before `deduceOptimalExtents` | Both matrices |
+| A9 | MATMUL_TRANSPOSE | `ext(a,0) == ext(b,0)` | `MATMUL-TRANSPOSE: unacceptable operand shapes (%jdx%jd, %jdx%jd)` (`RT/matmul-transpose.h:201-209`) | same | Both matrices |
+| A10 | MASK of SUM, PRODUCT, MAXVAL, MINVAL, MAXLOC, MINLOC | `!present(mask) \|\|` for each `d`: `ext(mask,d) == ext(array,d)`, with the extents read inside `fir.if present` | `Incompatible array arguments to %s: dimension %jd of ARRAY has extent %jd but MASK has extent %jd` (`RT/tools.cpp:97-104`) | `SHI` `ReductionAsElementalConverter::convert` (`:1142-1266`); `SI` `simplifyMinMaxlocReduction` | MASK |
+| A11 | Non-constant DIM on a rank-1 argument of ALL, ANY, COUNT, MAXLOC, MINLOC | `dim == 1` | `%s: bad DIM=%jd for ARRAY with rank 1` (`RT/reduction.cpp:256`, `RT/tools.cpp:345`) | `SHI` `AllAnyAsElementalConverter`, `CountAsElementalConverter`, `MinMaxlocAsElementalConverter`; `SI` `simplifyLogicalDim1Reduction`, `simplifyMinMaxlocReduction` | None (DIM is not used for addressing) |
+
+Not lost:
+
+- Partial reductions with a non-constant DIM are not inlined (`getConstDim`).
+- A constant DIM is checked by semantics.
+- MAXLOC and MINLOC with a BACK argument are not inlined by HLFIR.
+- FIR-level SUM, MAXVAL and COUNT only fire without DIM (and, for SUM and
+ MAXVAL, without MASK).
+- For SUM, PRODUCT, MAXVAL and MINVAL on rank-1 arrays, the runtime path already
+ ignores DIM, so an assert there would be a new check (kind `argument`).
+
+#### Transformational intrinsics
+
+| ID | Operation | Condition | Runtime message | Inserted in | Guard |
+|---|---|---|---|---|---|
+| A12 | CSHIFT and EOSHIFT with an array SHIFT | for each `k`, with `j` the matching ARRAY dimension (skipping DIM): `ext(shift,k) == ext(array,j)` | `%s: on dimension %jd, SHIFT= has extent %jd but ARRAY= has extent %jd` (`RT/transformational.cpp:48-53`) | `SHI` `ArrayShiftConversion`, after the ARRAY extents are computed, before `hlfir.elemental` or `hlfir.eval_in_mem` | SHIFT |
+| A13 | EOSHIFT with an array BOUNDARY | `!present(boundary) \|\|` same per-dimension test | `EOSHIFT: BOUNDARY= has extent %jd on dimension %jd but must conform with extent %jd of ARRAY=` (`RT/transformational.cpp:630-643`) | same | BOUNDARY |
+| A14 | EOSHIFT on CHARACTER with BOUNDARY | `len(boundary) == len(array)` | `EOSHIFT: BOUNDARY= has element byte length %jd, but ARRAY= has length %jd` (`RT/transformational.cpp:625-629`, `:700-704`) | same; the inline path pads or truncates instead of failing | None |
+| A15 | RESHAPE: SHAPE values | for each `i`: `shape(i) >= 0` | `RESHAPE: bad value for SHAPE(%jd)=%jd` (`RT/transformational.cpp:818-821`) | `SHI` `ReshapeAsElementalConversion`, after the SHAPE loads, before `fir.shape` (`:3130-3137`) | SHAPE extents fed to `fir.shape` |
+| A16 | RESHAPE: enough elements | `product(shape) <= size(array) \|\| (present(pad) && size(pad) > 0)` | `RESHAPE: not enough elements, need %jd but only have %jd` (`RT/transformational.cpp:831-836`) | SOURCE and PAD, and the dynamic extents used as divisors |
+
+A16 also prevents an unsigned division by zero in the inline index
+computation when ARRAY or PAD is empty (`SHI:3262-3266`). The divisions do
+not access memory, so guarding SOURCE does not order them: the extents they
+divide by are guarded too. Constant extents need no guard, because MLIR
+already treats a division by a non-zero constant as speculatable.
+
+Not lost: TRANSPOSE (rank only, static), character comparison
+(`hlfir.cmpchar`), and INDEX. Their runtime entry points have no user-facing
+checks.
+
+### 5.2 Checks missing in code generated inline during lowering
+
+| ID | Kind | Operation | Condition | Message | Inserted in | Guard |
+|---|---|---|---|---|---|---|
+| B1 | `status` | ALLOCATE of an allocated allocatable ([#223287](https://github.com/llvm/llvm-project/pull/223287)) | `null(box)` | `the object '%s' is already allocated` (runtime stat text: `Base address is not null`) | `flang/lib/Lower/Allocatable.cpp`, `AllocateStmtHelper`, before `genInlinedAllocation` (`:562-574`) | None needed (the program terminates before the leak is observable) |
+| B2 | `status` | DEALLOCATE of an unallocated allocatable | `!null(box)` | `the object '%s' is not allocated` (runtime: `Base address is null`) | same file, inline branch of `genDeallocate`, before `genFreemem` (`:1015-1020`) | None needed (`free(NULL)` is harmless) |
+| B3 | `divide` | Integer MOD/MODULO, `P==0` | `p != 0` | `MOD with P==0` / `MODULO with P==0` | `IC` `genIntegerZeroDivisorCheck`: replace `fir.if`; also add it to the UNSIGNED path (`genMod`, `genModulo`), which has none | `p` (the `arith.remsi` operand) |
+| B4 | migrate | NEAREST `S==0`, IEEE `RADIX/=2`, STORAGE_SIZE of an unallocated polymorphic, realloc with scalar RHS | existing conditions | existing messages | `IC` `genNearest`, `checkRadix`, `genStorageSize`; `MutableBox.cpp` `genReallocIfNeeded` | Operand of the protected op |
+| B5 | `bits` | BTEST, IBCLR/IBSET, IBITS, ISHFTC, MASKL/MASKR, MVBITS (out of range gives poison); ISHFT and SHIFTL/SHIFTR/SHIFTA/DSHIFTL/DSHIFTR (clamped, so silently accepted) | `0 <= pos < bit_size`, `pos + len <= bit_size`, and similar | gfortran `-fcheck=bits` style | `IC` per intrinsic | Shift or position operand |
+| B6 | `conformance` | Elemental operations and elemental procedure references on arrays | for each `d`: `ext(x,d) == ext(s,d)` for every array operand `x`, where `s` is the operand providing the iteration space | New, no runtime equivalent | `X2H` binary and unary elemental operations (`:1838-1886`), elemental calls in `flang/lib/Lower/ConvertCall.cpp` (`:2440-2472`); both carry a `TODO: merge shape` | Operands other than `s` |
+
+B1 and B2 are required by the standard and should be unconditional. The
+runtime path already checks them (`RT/allocatable.cpp:142-143`, `:198-199`);
+`-use-alloc-runtime` works around the problem today.
+
+B6 is new behavior at every optimization level: the runtime is never
+involved, and Flang checks no elemental expression today. gfortran only
+checks it under `-fcheck=bounds`, and ifx under `-check shape`. Whether it
+belongs to `conformance` or to a separate opt-in kind is open (section 9).
+The choice does not affect the other checks, which protect their own
+accesses. An elemental-call argument with dynamic OPTIONAL handling
+(`ConvertCall.cpp:2452-2471`) may be absent, so its condition must include
+presence.
+
+Also note: the CSHIFT shift normalization calls `genModulo` with the extent
+hidden behind a `select`, so with `-fcheck-integer-mod-zero-divisor` every
+CSHIFT gets a dead `MODULO with P==0` check (`SHI` `normalizeShiftValue`).
+Constant folding of the assert, or emitting `arith.remsi` directly there,
+fixes it.
+
+### 5.3 New opt-in checks (`bounds`, `pointer`, `do`)
+
+`inb(i, lb, ext)` is the single unsigned test `(i - lb) <u ext`. It is
+equivalent to `lb <= i <= lb + ext - 1` and false when `ext == 0`.
+
+| ID | Kind | Check | Condition | Inserted in | Guard | Notes |
+|---|---|---|---|---|---|---|
+| C1 | `bounds` | Array element subscript | for each `d`: `inb(i_d, lb_d, ext_d)`; for the last dimension of an assumed-size array, only `i >= lb` | `X2H` `HlfirDesignatorBuilder::visit(ArrayRef)`, scalar-subscript branch, after `genSubscript` (`:737-743`) | Subscript value | Skip constant subscripts (checked by semantics). Keep the legacy `dimension(1)` dummy idiom working. |
+| C2 | `bounds` | Section triplet `lo:hi:st` | `st != 0 && (n == 0 \|\| (inb(lo) && inb(last)))`, `last = lo + (n-1)*st` | same function, triplet branch, before `Triplet{lb, ub, stride}` (`:713-736`) | `lo`, `hi`, `st` | Check `last`, not `hi` (`a(1:11:3)` is valid for `ub=10`). Zero-size sections are exempt. |
----------------
eugeneepshteyn wrote:
(This comment was generated with the help of AI)
The check's condition has to be well defined on the input it rejects, and C2 is placed after lowering has already divided by the stride: the triplet branch of `HlfirDesignatorBuilder::visit(ArrayRef)` computes the section extent with `FirOpBuilder::genExtentFromTriplet` (an `arith.divsi` by `st`) before `Triplet{lb, ub, stride}`. With the extent computed first, LLVM treats `st == 0` as UB and folds the dominated `st != 0` branch away — the hazard described at lines 297-298. Guard the stride first and derive the extent from the guarded value. The same rule applies to the default-on checks: A13/A16 should build `present(x) && size(x)` with `fir.if` (as the A10 example does) rather than `arith.andi`, and A16's `product(shape)` must not use `nuw` multiplications, since branching on poison is UB.
```fortran
program p
real :: y(10), s
integer :: k
read *, k ! 0
y = [(real(i), i=1,10)]
s = sum(y(1:10:k)) ! flang -fc1 -emit-hlfir: divsi by k before the designate
print *, s
end program
```
https://github.com/llvm/llvm-project/pull/228502
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